This invention relates to a process for treating waste water contaminated with fatty residue. Water used in processing materials or cleaning equipment for or from vegetable or animal oil refining, fat rendering, tall oil processing, etc., is a typical waste water for treatment by use of this invention. Ordinarily the concentration of the fatty residue in such waste water is quite minor, typically of the order of 5,000-20,000 ppm, although it can be even higher or lower than this.
Fatty residue herein is meant to include fatty or fat-forming substances such as C.sub.4.sub.-26.sub.+ fatty acids, esters of such acids, especially glycerol and glycol esters, their soaps, their corresponding alcohols, and, in tall oil processing, some of the foregoing mixed with resin acids (abietic, pimaric, etc.), their soaps and/or esters. It is desirable to remove virtually all of such fatty residue from the waste water rapidly and cheaply as at least one stage of an effluent improvement scheme. Then, subsequent treatments, particularly biological ones, can be used effectively without overtaxing them.
Density separations such as with settling or skimming tanks have been used for many years for separating lighter oily phases from denser, preponderantly aqueous ones. The oil phase separation generally is quite good and sharp, but the preponderantly aqueous phase usually is still comparatively high in the fatty residues. Dissolved air flotation procedures are of more recent vintage. A permanent gas such as air (for efficiency and economy) is incorporated into the waste water under substantial pressure, then the pressure released to generate a froth. The froth is enriched in the fatty residue contaminants. Principles of this technique are described in the text, "Recent Developments in Separation Science," Volume 1, pages 113-127 (1972), edited by N. N. Li, CRC Press, Cleveland, Ohio. This disclosure is incorporated by reference in this patent application.
The instant process is especially useful in commercial operations where fairly large amounts of waste water are to be handled and treated continuously to quite a low fatty residue limit. Its advantages over prior proposals include high water throughout coupled with a high degree of fatty residue collection. In essence, the instant invention quite effectively tends to maximize the efficiency of the density separation and the dissolved air flotation techniques. The flow capacity of the dissolved air flotation unit is significantly increased by such unit's being limited to working on water that already has been submitted to density separation of some of the oily phase, and furthermore, the handling of the froth in the dissolved air flotation unit is greatly facilitated because no interfacial layer need be coped with and recycling condenses the awkward froth. The output of the previous density (skimming or the like) separation of a lighter oily phase is collected in a conventional efficient manner, eg. by decanting and/or scraping; the denser separated aqueous phase, having been rapidly treated to such "coarse" separation, need not be monitored carefully because it is given a second stage purification having a fair tolerance for more or less fatty residue still present.